Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine. Thus, turbo charging can extend the operating region of an engine. Turbochargers function by compressing intake air in a compressor via a turbine operated by exhaust gas flow. Under certain conditions, the flow rate and pressure ratio across the compressor can fluctuate to levels that may result in noise disturbances, and in more severe cases, performance issues and compressor degradation.
Such compressor surge may be mitigated by one or more compressor recirculation valves (CRV) located in the intake. The CRVs may recirculate compressed air from the compressor outlet to the compressor inlet. However, at some operating conditions, opening the CRV to mitigate noise issues may result in insufficient boost to meet current torque requests. Particularly, this may result in poor response in transient conditions, such as following a partial throttle tip-out.
The inventors have recognized the issues with the above approach and offer a system to at least partly address them. In one embodiment, an engine system comprises an engine, first and second compressors supplying air to the engine, a first compressor recirculation valve adjustable to two restriction levels, and a second compressor recirculation valve adjustable to three or more restriction levels.
In this way, the second, variably adjustable compressor recirculation valve may be operated to provide an adjustable amount of intake flow that recirculates through the compressor. The first compressor recirculation valve, which is a traditional on/off compressor recirculation valve, may be operated to alleviate compressor surge, for example, under high load conditions. Together, compressor surge may be mitigated by concerted control of both valves, leading to reduced noise disturbances while providing acceptable boost in transient conditions. Additionally, in engine systems that include a single turbocharger, the two compressor recirculation valves may be arranged in parallel to recirculation the one compressor, and controlled to mitigate surge.
The present disclosure may offer several advantages. For example, an engine that includes both the first and second compressor recirculation valves as described may be extensively downsized to improve engine efficiency and fuel economy, as the variably adjustable valve allows for engine operation close to the surge boundary while avoiding surge and providing sufficient boost. Boost under transient conditions may also be improved, as the variably controlled compressor recirculation valve allows for proper boost for adequate torque response following, for example, a sudden drop in engine load during an operator tip-out event.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.